if its an OMC stringer drive, then you are SOL on a 4.3 unless its a 1985 only 4.3 with a 2-piece RMS crank as there are no couplings available for the stringer drive and a 1-piece RMS crank. if you find the ford coupling you should be able to have the OD and pilot and bolt pattern re-machined
if its a mercruiser, then no problem if you follow Chris's comment.